1. Field of the Invention
The present invention relates to impregnated electric wound capacitors.
2. Description of the Prior Art
Impregnated electric wound capacitors are known in the art, such as German Auslegeschrift 18 15 478, which have a dielectric consisting of swollen polypropylene foils impregnated with mineral oil and whose electrodes consist of paper tapes metalized on both sides and arranged in the field-free space. The polypropylene foils of the capacitor generally exhibit a minimum swelling of 0.3 .mu.m. The swelling of the foils is accordingly dimensioned in such a manner that practically no residual impregnation gap is present in the winding after the swelling is completed.
The absence of a residual impregnation gap means that re-impregnation of the winding is no longer assured after the winding is subjected to stresses of various types. Such stresses may be the result the temperature changes during operation, or a gas pocket which may arise due to a partial blow-out. These stresses produce hollow spaces in the winding at which partial discharges can occur when no impregnation agent can penetrate into the hollow spaces to re-impregnate the polyproplylene foils. Moreover, a particular problem arises in connection with increased temperature, because the pressure in the winding can increase to such an extent that the foils or the paper tapes may rip, thereby destroying the capacitor.